Powder coating is a technique of applying dry paint, in the form of a powder, to a substrate. Powder coating is frequently used for the coating of metal products such as automobile parts, as well as for the coating of substrates of different materials.
In order to color a substrate with a powder, the powder is usually first applied to the substrate by using a spray gun, an electrostatic disc, a magnetic brush, or by simply dipping the substrate into the powder. The powder may be imparted with an electrostatic charge, so that when the substrate is electrically grounded or charged negatively, the charged powder adheres to the substrate. Then, heat may be applied to the powder and/or to the substrate, commonly in the range of 100-250° C. The heat causes melting of the powder and binds it to the substrate. Then, the powder may be allowed to cool down and cure. The cured, melted powder forms a coating on the substrate.
Powder coating is usually produced by mixing one or more base powders containing a polymeric melting member with one or more pigments. The mixture is heated and melted in an extruder to form a colored melt. In the melting process, particles of the base powders and the pigments mix essentially uniformly. The colored melt is cooled down and processed by the extruder into various forms such as chips or pellets. The extruded product is then fed into a grinder and is ground into a color powder essentially ready to be applied. This color powder commonly includes particles which are agglomerates of both the base powder and the pigments. That is, the extrusion process is capable of blending the base powder and the pigments in a highly uniform manner.
Although an extrusion process has the advantage of creating a highly uniform color powder, it is said to have some major drawbacks in the form of costs and efficiency. An extruder is a highly expensive machine, with some extruders being retailed in the range of hundreds of thousands of dollars. In addition, a single extrusion process creates a powder of a single color—the outcome of the pigments and base powder(s) fed into the extruder. This color normally cannot be changed later, especially not by an end-user who purchased the color powder with the intention of applying it to a substrate. An industrial extruder usually has a relatively high capacity, often in the range of half a ton to one ton. Therefore, if a color powder manufacturer is being requested to produce a smaller quantity of a certain color powder, the manufacturing may not be economical. Extrusion is said to be suitable for relatively large quantities of color powder, in the range of 0.5-1 ton capacity of a typical industrial extruder.
U.S. Pat. No. 5,319,001 to Morgan et al. discloses a process for the preparation of a colored powder coating composition. PCT Publication no. WO2007/050417 to O'Dell et al. discloses a method for coloring powders, including mixing a base powder and non-incorporated pigments. U.S. Pat. No. 5,856,378 to Ring et al. discloses a powder coating composition which includes composite particles that are agglomerates of individual particulate components fused or bonded together. U.S. Pat. No. 6,113,979 to Sagawa et al. discloses a powder coating used in a method for forming a coating on the surface of materials.